The research to be carried out under this supplement to grant HL15728 has logically developed as an extension of our studies on the carbohydrate of the blood-clotting glycoproteins. In these studies we propose to accurately determine the properties of protein-phospholipid interactions which are a part of the blood coagulation reactions as a function of phospholipid composition, calcium (or other cation) concentration and protein concentration. The techniques used are fluorescence, fluorescence energy transfer and light scattering. These studies will originally examine prothrombin and factor X but will subsequently involve other blood-clotting proteins. The end result should allow the calculation of the quatity of each blood-clotting protein which is bound to a membrane of any (known) phospholipid composition under any combined condition of cation or protein concentration or temperature. From these findings we hope to be able to reconstruct the actual zymogen-activating complexes of the blood clotting system and understand the conditions for optimal reactions.